The present invention relates to a method for changing over to a different frequency at a cellular phone system, and a cellular phone system using the method, and a base station controlling apparatus in the system, and a mobile communication terminal in the system, in particular, in which the method for changing over to a different frequency is improved at a CDMA (code division multiple access) system.
Description of the Related Art
At a cellular phone system using a CDMA system, in order to keep the channel capacity in the high level, its transmission power is controlled. Generally, the transmission power is controlled by using a value of SIR (signal to interference power ratio) showing in the following equation (1).SIR=S/(I+N)  (1)
In this, the S signifies desiring wave signal power, the I signifies interference power per band, and the N signifies noise power per band.
At the control of the transmission power, the reception side measures the reception SIR shown in the equation (1). When the measured result is smaller than a target SIR, the reception side requires the transmission side to increase the transmission power, and when the measured result is larger than the target SIR, the reception side requires the transmission side to decrease the transmission power. With this, its channel quality can be controlled in a certain value and the channel capacity can be kept in the high level.
And at the cellular phone system using the CDMA system, a soft hand-over technology, in which a mobile communication terminal connects to plural base stations at the same time, is important. At the soft hand-over technology, a mobile communication terminal cyclically measures reception power in broadcast channels transmitting from base stations in a cell, where the mobile communication terminal exists, and from a base station in an adjacent cell. And the maximum value in the measured reception power in the broadcast channels is made to be a reference, and the mobile communication terminal connects its channels to base stations that transmitted the broadcast channels whose reception power is within a threshold value from the maximum value (reference). With this, the mobile communication terminal connects to the plural base stations at the same time. In this, hereinafter, this threshold value is referred to as an active set threshold value.
In case that a mobile communication terminal is in a position near a base station, the propagation loss between the mobile communication terminal and the base station is small enough, compared with the propagation loss between the mobile communication terminal and a base station in an adjacent cell. And the reception power in the broadcast channel transmitting from the base station near the mobile communication terminal becomes the maximum value. And the difference between the reception power in the broadcast channels from the base station near the mobile communication terminal and the base station in the adjacent cell becomes larger than the active set threshold value. Therefore, the mobile communication terminal connects its channel only to the base station near the mobile communication terminal.
In case that a mobile communication terminal is in a position at the end of a cell of a base station, the propagation loss between the mobile communication terminal positioned at the end of the cell and the base station and the propagation loss between the mobile communication terminal and a base station in an adjacent cell become almost equal. Therefore, the difference of the reception power in the broadcast channels transmitting from the base station positioned at the end of the cell and from the base station in the adjacent cell becomes small. Consequently, the mobile communication terminal receives plural broadcast channels within the active set threshold value, and the mobile communication terminal connects its channels to the plural base stations at the same time. As mentioned above, by using the soft hand-over technology, the channel of the mobile communication terminal does not disconnect even while the mobile communication terminal is moving from a cell to an adjacent cell.
Further, at the cellular phone system using the CDMA system, in case that plural carrier frequencies are allocated, a technology, which changes over its carrier frequency to a different carrier frequency, is important.
Generally, this technology for changing over to the different frequency is a technology to make the frequency utilization efficiency high, and has a purpose that a call drop phenomenon caused by the deterioration of the channel quality in the currently using carrier frequency is avoided.
First, the technology changing over to the different frequency is explained in the viewpoint of the system structure. In the explanation of prior arts, drawings showing in FIGS. 1 to 4, which are the drawings using at embodiments of the present invention, are used.
In FIG. 1, the structure of a cellular phone system, in which two carrier frequencies are used in one service area, is shown. In FIG. 2, the positions of the carrier frequencies in the upstream channel and the downstream channel are shown.
Base stations 10, 11, 20, and 21, mobile communication terminals 30, 31, and 32, and a base station controlling apparatus 60, which controls the base stations, are shown in FIG. 1.
In FIG. 1, the mobile communication terminal 30 can connect its channel to any of base stations 10, 11, 20, and 21. The carrier frequencies, which the mobile communication terminal 30 uses in the channel with the base stations 10 and 11, is a carrier frequency 101 at the upstream channel, and a carrier frequency 103 at the downstream channel shown in FIG. 2. And the carrier frequencies, which the mobile communication terminal 30 uses in the channel with the base stations 20 and 21, is a carrier frequency 102 at the upstream channel and a carrier frequency 104 at the downstream channel shown in FIG. 2.
And the judgement changing over to the different frequency, whether the mobile communication terminal 30 connects to the base stations 10 and 11 or the base stations 20 and 21 during its communication, is controlled by the base station controlling apparatus 60 or the mobile communication terminal 30. And also the base station controlling apparatus 60 or the mobile communication terminal 30 executes the control changing over to the different frequency. In FIG. 1, broadcast channels 40 and 41, downstream individual channels 50 and 51, and upstream individual channels 70 and 71 are also shown.
Next, a conventional method for changing over to a different frequency is explained. At this conventional method for changing over to the different frequency, the control changing over to the different frequency is executed corresponding to the reception quality at the broadcast channels.
In FIG. 1, it is assumed that the channel of the mobile communication terminal 30 has been connected to the base station 10 as the initial state. At this time, at the downstream channel of the mobile communication terminal 30, the currently using carrier frequency is 103, and the different carrier frequency to which the currently using carrier frequency is changed over is 104. And at the upstream channel, the currently using carrier frequency is 101, and the different carrier frequency to which the currently using carrier frequency is changed over is 102.
At this time, the mobile communication terminal 30 measures the reception quality Q_1 in the broadcast channel of the currently using carrier frequency 103 and the reception quality Q_2 in the broadcast channel of the different carrier frequency 104 to which the currently using carrier frequency is changed over, and compares the measured results. And at the time when the difference between the reception quality Q_1 and Q_2 satisfies the following inequality (2), the changing over to the different frequency is executed.Q—2−Q—1>Th—HO [dB]  (2)
The changing over to the different frequency signifies that a carrier frequency using currently is changed over to a different frequency. In this case, at the downstream channel, the carrier frequency is changed over from the carrier frequency 103 to the carrier frequency 104, and at the upstream channel, the carrier frequency is changed over from the carrier frequency 101 to the carrier frequency 102.
At the inequality shown in (2), the Th_HO signifies a judging threshold value at the control for changing over to a different frequency, and the value of the Th_HO is positive.
Generally, at the inequality (2), the smaller the value of the Th_HO is, the smaller the probability generating a call drop becomes. However, the load controlling the changing over to the different frequency is increased when the value of the Th_HO is set to be small. Therefore, an optimum value is set as the value of the Th_HO. By the description mentioned above, the conventional method for changing over to the different frequency corresponding to the reception quality in the broadcast channels was explained.
At the cellular phone system using plural carrier frequencies, generally the mobile communication terminal in the system uses one oscillator from the viewpoint of low power consumption, and the oscillating frequency of the oscillator is changed, when it is needed. Under this structure of the mobile communication terminal, when the reception quality of the different carrier frequency, to which the currently using carrier frequency is changed over, is measured, there is a following problem due to the one oscillator. That is, the mobile communication terminal cannot receive data from the currently using carrier frequency while the mobile communication terminal is measuring the reception quality of the different carrier frequency to which the currently using carrier frequency is changed over.
Therefore, at the system, it is necessary that its transmitting waveform has been formed, at the state that a data vacant time for measuring the reception quality of the different carrier frequency to which the currently using carrier frequency is changed over is kept beforehand.
In case that the communication speed is a constant, in order to make the data vacant time, a data compression technology, by which the data vacant time is made, is required.
Generally, at the data compression technology, the transmission data are compressed in the time by using a method lowering its diffusion rate or a method making its coding rate higher by that a part of coded data is not transmitted. At the data compression technology, when it is compared with a case in which the data compression technology is not used, the error correction ability at the reception is deteriorated, therefore the frequency utilization efficiency is lowered. Consequently, it is not desirable that the ratio of the data vacant time to the communication time becomes high. Hereinafter, the ratio of the data vacant time to the communication time is referred to as the ratio of the data vacant time.
By the reason mentioned above, at the time of measuring the different frequency, it is desirable that the timing of changing over to the different frequency is judged accurately in a low ratio of the data vacant time. In order to lower the ratio of the data vacant time, the following improved control method is used for the conventional method for changing over to the different frequency mentioned above.
Next, this improved method, in which the range of the measurement of the different frequency is limited, is explained. In FIG. 3, at the downstream channel to the mobile communication terminal 30, the waveform of the reception quality Q_1 of the broadcast channel in the currently using carrier frequency and the waveform of the reception quality Q_2 of the broadcast channel in the different carrier frequency to which the currently using carrier frequency is changed over are shown.
In FIG. 3, in order to decrease the data vacant time at the time when the different frequency (the reception quality Q_2) is measured, the mobile communication terminal 30 starts to measure the different frequency (the reception quality Q_2) at the time t1 or t3, when the reception quality Q_1 of the broadcast channel in the currently using carrier frequency satisfied the following inequality (3).Q—1<Th_Start  (3)
After this, the mobile communication terminal 30 ends the measurement of the different frequency at the time t2, when the reception quality Q_1 of the broadcast channel in the currently using carrier frequency satisfied the following inequality (4). Or the mobile communication terminal 30 ends the measurement of the different frequency at the time t4, when the inequality (2) was satisfied, at the same time the control of the changing over to the different frequency is started.Q—1>Th_End  (4)
In this case, the judging threshold values Th_Start and Th_End in the inequalities (3) and (4) are desirable to have a margin satisfying the following inequality (5).Th_End−Th_Start>0 [dB]  (5)
In the inequality (5), the larger the difference between the Th_End and Th_Start is, the larger the margin becomes. By the concept mentioned above, the range of measuring the different frequency is limited by using the judging threshold values Th_Start and Th_End. With this, the ratio of the data vacant time can be decreased, compared with the case in which the range of measuring the different frequency is not limited.
Generally, when the margin becomes large, the number of times of start and end of the measurement of the different frequency is decreased, and there is an advantage that the controlling load at the base station controlling apparatus 60 is decreased. However, since the number of times of the end of the measurement of the different frequency is decreased, the ratio of the data vacant time is increased, and the channel capacity is decreased. Therefore, the margin at the inequality (5) must be an optimum value.
Next, a cellular phone system showing in FIG. 4 is studied. In FIG. 4, the mobile communication terminal 30 can connect its channel to any of the base stations 10, 11, and 20. The carrier frequencies, which the mobile communication terminal 30 uses in the channel with the base stations 10 and 11, is a carrier frequency 101 at the upstream channel, and a carrier frequency 103 at the downstream channel shown in FIG. 2. And the carrier frequencies, which the mobile communication terminal 30 uses in the channel with the base station 20, is a carrier frequency 102 at the upstream channel and a carrier frequency 104 at the downstream channel shown in FIG. 2.
And the judgement changing over to the different frequency, whether the mobile communication terminal 30 connects to the base stations 10 and 11 or the base station 20 during its communication, is controlled by the base station controlling apparatus 60 or the mobile communication terminal 30. And also the base station controlling apparatus 60 or the mobile communication terminal 30 executes the control changing over to the different frequency.
In FIG. 4, the base station 10 belongs to a macro-cell 85 and the base station 20 belongs to a macro-cell 87, and the base station 11 belongs to a micro-cell 86. At the cellular phone system shown in FIG. 4, in which a micro-cell is added in a macro-cell, by disposing the micro-cell in an area, where many mobile communication terminals exist, in the macro-cell, an effect dispersing the traffic can be obtained. Therefore, this is one of the methods for designing effective cells.
At the conventional technology, as mentioned above, in the inequality (2), the relative value (Q_2−Q_1) between the reception quality Q_1 of the broadcast channel in the currently using carrier frequency and the reception quality Q_2 of the broadcast channel in the different frequency to which the currently using carrier frequency is changed over is compared with the judging threshold value Th_HO. In this case, the judging threshold value Th_HO in the inequality (2) is a constant value being not relating to the moving velocity of the mobile communication terminal. Therefore, there is the following problem at the cellular phone system shown in FIG. 4.
At the conventional technology, when the moving velocity of a mobile communication terminal connecting to a micro-cell becomes high, the probability that the mobile communication terminal moves out of the area of the micro-cell becomes high. Consequently, the probability deteriorating the reception quality of the channel becomes high, and the probability generating a call drop becomes high. Further, in case that a micro-cell cellular is composed of plural micro-cells and a macro-cell exists in a state that the micro-cell cellular is covered with the macro-cell, when the moving velocity of a mobile communication terminal connecting to a micro-cell becomes high, the number of times of changing over to different micro-cells by the mobile communication terminal becomes large. Consequently, the load changing over to the different micro cells becomes large.
Therefore, at the conventional technology, there is a problem that it is difficult to decrease the probability generating the call drop and to decrease the load changing over to the different micro-cells when the moving velocity of a mobile communication terminal connecting a micro cell becomes high.
Further, at the method for changing over to a different frequency in the conventional technology, a case, in which the moving velocity of a mobile communication terminal whose channel is connecting to a macro-cell becomes low and the mobile communication terminal stays in the area of a micro-cell, is assumed. In this case, the transmission power from the mobile communication terminal whose channel is connecting to the macro-cell to a base station in the macro-cell and the transmission power from the base station in the macro-cell to the mobile communication terminal exist. And the transmission power mentioned above is compared with the transmission power in a case that the mobile communication terminal changes over its channel to a micro cell. In this case, there is the probability that the transmission power in the macro-cell becomes higher than that in the micro-cell. When the transmission power becomes large, the power consumption also becomes large. Further, when the transmission power becomes large, the interference power to other cells is increased, and the call drop ratio becomes large.